1. The Field of the Invention
The present invention relates to systems and methods for compensating timing jitter. More particularly, the present invention relates to systems and methods for digitally compensating for timing jitter in an optical transmitter.
2. The Relevant Technology
Lasers and other light emitting devices are important components of many optical systems. The light generated by lasers can be modulated to carry data over fiber optic networks at increasingly faster rates. Although light signals are used to carry data over fiber optic networks, the data carried by the light signals often originates as an electrical signal and the conversion of an electrical signal into a light signal is usually accomplished using an optical transmitter.
Presently, commercially available optical systems use direct on-off keying (OOK) modulation. In such a system, the transmission path of an optical transmitter usually includes a laser driver that modulates the laser current, thereby changing the intensity of the laser light according to the data in the electrical signal. The intensity of the light signal corresponds to the 1's and 0's in the digital data stream. High intensity light represents digital 1's while low intensity light represents digital 0's.
The successful generation and transmission of light signals can depend on the ability of the optical transmitter to convert a high speed electrical data stream into an optical data stream. One of the problems associated with the generation and transmission of light signals in an optical transmitter is jitter. Jitter becomes more problematic as the transmission speed increases. Jitter is often variable and can be generated by a variety of different sources such as impedance mismatch and the bandwidth limitation of the driver and the associated packaging and interconnections, etc. A directly modulated laser produces additional jitter due the laser characteristics itself. The physics of the laser is typically described by the rate equations from which the frequency response and the transient response of the laser to an electrical stimulus can be determined.
As the operating current and the temperature of the laser change, the frequency and the damping factor of the laser's relaxation oscillation can change and have an impact on the settling time of the laser. As the speed of transmission increases, the settling time of this response becomes more important. If the settling time of a laser is more than a single bit period, then jitter will be produced when a transition occurs. Specifically, settling time longer than a single bit period can result in transitions that occur too soon or too late from the ideal position in time. In other words, laser jitter is experienced.
Laser jitter may also have the characteristic of being non-linear. The jitter associated with rising transitions, for example, may be different than the jitter associated with falling transitions. Laser jitter may also be affected by earlier transitions in the data stream. It should also be noted that even though the laser exhibits non-linear jitter characteristics, the jitter may not be random but caused by a particular occurrences of the data sequence. This type of jitter is well known in the industry and it is called deterministic or systematic jitter.